Titanium aluminide, cast made therefrom and method of making the same

ABSTRACT

TiAl alloy includes 46 to 50 at % of Al, 5 at % or less of combination of Mo, V and Si, provided that Si content is 0.7 at % or less, and Mo content satisfies an equation of −0.3x+17.5 at % or less where x represents Al (at %), and the remainder being Ti and inevitable impurities. Mo may be replaced by Fe or combination of Mo and Fe. TiAl alloy is heated to a melt, poured into a mold, and cooled at a rate of 150 to 250° C./min within a temperature range of 1500 to 1100° C. The resulting product can be used as cast. If desired, however, heat treatment such as HIP or homogenization may be performed within a temperature range of 1100 to 800° C. After the heat treatment, the melt is cooled at a rate of 100° C./min or more until room temperature.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to titanium aluminide,cast (or mechanical part) made from the titanium aluminide, and methodof making the cast, and more particularly relates to those used inmanufacture of mechanical parts of a turbocharger mounted on a dieselengine operating under an elevated temperature for a long period.

[0003] 2. Description of the Related Art

[0004] Titanium aluminide is an alloy of Al and Ti. Because of itscharacteristics such as lightweight and high strength, TiAl is commonlyused in rotating parts of jet engines and automobile engines. When TiAlis used in mechanical parts of a vehicle such as parts of a turbochargerof a diesel engine, which are subjected to a very high temperature for aconsiderable time of period, however, additional considerations andimprovements are needed in terms of mass productivity, costeffectiveness, creep resistance, oxidation resistance, etc.Specifically, mechanical parts made from conventional TiAl are mostlyfabricated by forging, but the forging process is not suited for massproduction. Since automobiles are made in a large number, it is notpractical to manufacture the parts of the turbocharger by the forgingprocess.

[0005] In the meantime, it is known that the creep resistance can beimproved by adding third and/or fourth element such as W, Ta, Nb and Cr.However, addition of the third/fourth element would greatly degradeprecision castability. The mechanical parts of the engine should oftenbe made by precision casting. It is also known that the creep resistancecan be raised by forging if the forging is performed in a manner tocontrol the structure. However, this requires complicated heattreatment, which in turn results in raised cost.

[0006] Further, the conventional TiAl is poor in oxidation resistanceunder high temperature. Specifically, the surface of the product isoxidized if the surrounding temperature exceeds 700° C., and theresulting scale peels off. Accordingly, the product made from theconventional TiAl cannot be used for the turbocharger or the like thatis designed to operate in an environment over 700° C.

SUMMARY OF THE INVENTION

[0007] An object of the present invention is to provide TiAl thatpossesses mass productivity, improved creep resistance and improvedoxidation resistance while maintaining preferred characteristics theabove-mentioned conventional TiAl already has.

[0008] Another object of the present invention is to provide a productcast from such TiAl.

[0009] Still another object of the present invention is to provide amethod of making such product.

[0010] According to one aspect of the present invention, there isprovided a TiAl alloy including:

[0011] Al: 46 to 50 at %;

[0012] Mo, V and Si: a total content of these elements being limited to5 at % or less, provided that Si content is 0.7 at % or less, and Mocontent satisfies an equation of −0.3x+17.5 at % or less where xrepresents Al content (at %); and

[0013] the remainder being Ti and inevitable impurities.

[0014] Each of Mo, V and Si should be included more than zero %. Mo maybe replaced by Fe, or combination of Fe and Mo.

[0015] TiAl alloy is heated to a melt, poured into a mold, and cooled ata rate of 150 to 250° C./min within a temperature range of 1500 to 1100°C. From 1100 to 600° C., the melt is preferably cooled in the moldnaturally or a cooling rate faster than natural since cracking wouldoccur in the cast if it is cooled too fast and a desired structure wouldnot result if it is cooled too slow. After 600° C., it may be cooled atan arbitrary rate.

[0016] The resulting product (cast) has additional characteristics suchas improved mass productivity, creep resistance and oxidation resistancein addition to inherent characteristics of TiAl such as lightweight andhigh strength. Specifically, the product is fabricated by casting, whichis suited for mass production. Conventionally, the product is fabricatedby forging. Addition of small amount of V improves castability. It isknown that the creep resistance is deteriorated when the β phase and/orcoarse silicide are precipitated in the mother material duringsolidification. By admitting an only small amount of Mo in TiAl alloy,however, such (coarse) precipitation can be prevented. Therefore, thecreep resistance is significantly improved in the TiAl alloy of theinvention. Inclusion of small amount of Si improves the oxidationresistance.

[0017] By controlling the melt cooling rate to 150 to 250° C./min in atemperature range of 1500 to 1100° C., the product (as cast) has a fullyor completely lamellar structure only. Accordingly, no heat treatment isrequired after the casting process. This contributes to reduction of amanufacturing cost.

[0018] Consequently, the product made from the TiAl of the invention bythe casting method of the invention has all of the followingcharacteristics: high strength, lightweight, high mass productivity,high creep resistance and high oxidation resistance. Since mechanicalparts of a turbocharger or jet engine must have such characteristics fortheir liability and practicability, the TiAl alloy of the invention andthe casting method are particularly suited for manufacture of theturbocharger or jet engine parts.

[0019] Although the as-cast product can be used immediately as amechanical part, heat treatment such as HIP or homogenization may beperformed later. Such heat treatment may be conducted within atemperature range of 1100 to 800° C. or T(° C.)≧{1200° C.+25(Al−44)}+10.The cooling rate after this heat treatment may be controlled to 100°C./min or more until room temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a graph illustrating relationship between an amount ofAl contained, and hardness of TiAl;

[0021]FIG. 2 is a graph showing relationship between an amount of Alcontained, elongation, and stress;

[0022]FIG. 3 is a phase diagram showing relationship between Al contentand temperature;

[0023]FIG. 4 is a ternary TiAl—Mo phase diagram at 1473K with comparisonof ternary TiAl—Fe phase diagram;

[0024]FIG. 5 is a copy of microphotograph showing precipitation of βphase on a lamellar grain boundary in a comparative example;

[0025]FIG. 6 is a copy of microphotograph showing a fully lamellarstructure in TiAl of the present invention;

[0026]FIG. 7 is a copy of microphotograph showing precipitation ofcoarse silicide when Si is added more than 0.7 at %;

[0027]FIG. 8 illustrates a Ti—Al phase diagram;

[0028]FIG. 9 illustrates results of a creep rupture test; and

[0029]FIG. 10 illustrates high temperature oxidation property of TiAl ofthe invention and TiAl of the prior art.

DETAILED DESCRIPTION OF THE INVENTION

[0030] Now, embodiments of the present invention will be described inreference to the accompanying drawings.

[0031] TiAl of the present invention includes 46 to 50 at % of Al; and 5at % or less of combination of Mo, V and Si, provided that Si is added0.7 at % or less, and Mo is added in an amount calculated by thefollowing equation: −0.3x+17.5 at % or less when x represents an amountof Al contained (at %), with the remainder being Ti and inevitableimpurities.

[0032] A product of the present invention is made from this TiAl.Specifically, this TiAl is melt and poured into in a mold. Then, themelt is cooled at a rate of 150 to 250° C./min in a temperature range of1500 to 1100° C. From 1100 to 600° C., it is preferably cooled in themold naturally or at a rate faster than natural since cracking wouldoccur in the cast if cooled too fast and a desired structure would notresult if cooled too slow. The product can be used as cast.

[0033] TiAl and the resulting product have improved characteristics suchas higher mass productivity, creep resistance and oxidation resistancein addition to inherent characteristics of TiAl such as lightweight andhigh strength. Specifically, even when the product is used as amechanical part in a turbocharger of a diesel engine operating at atemperature of 800° C. or more for a considerable period repeatedly, nocreep rupture and scale peeling would not occur. Further, after cooledto the room temperature in the mold (i.e., upon completion of thecasting process), the solidified TiAl can be used immediately withoutheat treatment, so that the product can be manufactured in a large massat a reduced cost. Moreover, the lightweight and high strength, whichare the original characteristics of TiAl, are adversely affected little.

[0034] Now, the composition of this alloy (TiAl) and a method of castingwill be described.

[0035] Al content of the alloy according to the present invention shouldfall within a range of 46 to 50 at %. In general, the product as casthas cracking in its surface or inside due to shrinkage duringsolidification. In order to prevent such cracking, the product should bedehardened and possess room temperature ductility. In the case of TiAl,as shown in FIGS. 1 and 2, TiAl has sufficient room temperatureductility when Al is contained 45.5 at % or more. However, when Al iscontained 45.5 at %, the oxidation resistance is low. Consequently, Alshould be included at least 46 at %. In order to raise the creepresistance at high temperature, on the other hand, the cast should havea fully lamellar structure with (or constituted by) α₂ (Ti₃Al) phase andγ (TiAl) phase. This structure is obtained when Al is contained about 38to 50 at % (see FIG. 3). In the present invention, therefore, Al contentis limited to 46 to 50 at % in order to have both appropriate roomtemperature ductility and fully lamellar structure.

[0036] The third and fourth elements to be added are a group of Mo, Vand Si, a group of Fe, V and Si, or a group of Mo, Fe, V and Si. Mo andFe are selectively included, both or one of them. One of these threegroups is included in TiAl of the invention, and the content of thegroup is limited to 5 at % or less. The combination of V, Si, Mo and/orFe serves to stabilize the β phase in the Ti alloy. In order to give thehigh temperature creep resistance to the TiAl cast, TiAl should possessthe fully lamellar structure of α₂+γ phase without the β phase. Inparticular, Fe and Mo are strong elements in terms of the β phasestabilization. As depicted in FIGS. 4 and 5, experiments have shown thatthe β phase is precipitated on the lamellar grain boundary even if Fe/Mois added in a trace amount, when Al is included in a range of 46 to 50at %. The resulting β phase degrades the high temperature creepresistance. In view of these facts and further experimental results on amicrostructure, the inventor concluded that the total amount of Si, V,Fe and/or Mo should be limited to 5 at % at most. The fully lamellarstructure obtained under the above condition is seen in FIG. 6.

[0037] It should be noted here that the amount of Si should be limitedto 0.7 at % or less. This is because addition of Si over 0.7 at % wouldresult in a coarse Si compound precipitated in the lamellar structure.This would likely become an origin of fatigue failure. Such possibilityis particularly undesirable to a machine having a rotating member suchas turbocharger. For comparison, silicide precipitated as a result ofadding Si over 0.7 at % is shown in FIG. 7.

[0038] It should also be noted that the upper limit of Mo content isdetermined by the following equation where x represents the amount of Al(at %): −0.3x+17.5 at %. The reason is because, as illustrated in FIG.4, the β phase does not precipitate if Mo is limited to such range. Thisis understood from the phase boundary between α+β+γ and α+γ, as well asmicro structure observation. For example, when Al content is 48 at %,the tolerable maximum value of Mo content is 3.1 at %. If Mo is includedover this value, the β phase is precipitated and the creep resistance isconsiderably deteriorated. It is satisfactory to substitute Fe for Mo,or further add Fe, in order to obtain the same result.

[0039] Immediately after pouring the melt of this TiAl into a mold, itis cooled at a rate of 150 to 250° C./min in a temperature range of 1500to 1100° C. This cooling rate is important to prevent the β phase fromprecipitating in the product as cast, i.e., to obtain the fully lamellarstructure having a complete binary (α+β) phase thereby providing highcreep resistance. If the cooling rate is below 150° C./min, it is notpossible to obtain a lamellar structure having small layer gaps. As theAl content approaches 50 at %, γ particles tend to appear in thelamellar structure. The slower the cooling speed, the greater mount theγ particles precipitate. On the other hand, if the cooling rate exceeds250° C./min, a cooling rate difference between the product surface andinterior may become very large. For example, when a part of aturbocharger is prepared by the casting at the cooling rate over 250°C./min, ductility cannot follow shrinkage upon solidification. Thiswould result in cracking upon casting. For example, when turbine partsare cast, cracking may occur in turbine vanes or their root portions.

[0040] If parts of a diesel engine turbocharger are fabricated from TiAlof the invention by the casting, the following ratio is preferred amongAl, Mo (Fe), V and Si, although it is ultimately determined according tothe size and operating conditions of the product: 48±1.0 at % of Al, 0.4to 0.8 at % of Mo (Fe), 0.5 to 1.1 at % of V, and 0.1 to 0.3 at % of Si.The cooling rate is preferably maintained to 150 to 250° C./min withinthe temperature range of 1500 to 1100° C.

[0041] The resulting cast can be immediately used as a product(mechanical part of the turbocharger). However, there is a possibilitythat some deficiencies may exist in the product since it is an as-castproduct which does not undergo any heat treatment. Accordingly, ifdesired or necessary, suitable heat treatment such as HIP(Hot IsostaticPress) or homogenization is applied to the cast to eliminate possibledeficiencies.

[0042] Heat treatment conditions should be determined in such a mannernot to destroy the fully lamellar structure formed in theabove-mentioned cooling process. Specifically, the heat treatment isperformed in a temperature range of 800 to 1100° C. Such coolingmaintains the fully lamellar structure and eliminates the castingdeficiencies. In order to maintain the fully lamellar structure obtainedby the cooling at the rate of 150-250° C./min in the casting processafter the heat treatment, the heat treatment temperature should be belowabout 1125° C., which is the eutectoid temperature. The inventorconsidered temperature variations/irregularity in industrialfurnaces/ovens and concluded that the practical upper limit temperatureis 1100° C. The lower limit temperature should be higher than a value atwhich the product is used (about 750° C.), and a value such that thehomogenization or HIP effect be fairly provided by the heat treatment.After experiments, the inventor concluded that the lower limittemperature is practically 800° C.

[0043] Alternatively, the heat treatment may be conducted in a rangesatisfying the following equation: T(° C.)≧{1200° C.+25(Al−44)}+10. Suchcooling also maintains the fully lamellar structure and eliminates thecasting deficiencies. The fully lamellar structure obtained by the150-250° C./min cooling in the casting process, which insuressatisfactory creep resistance at elevated temperature, should bemaintained even after the heat treatment. If the heat treatment isconducted in an area of α+γ, as shown in FIG. 8, then γ particles wouldprecipitate. Consequently, the fully lamellar structure is not obtained.To avoid such microstructural deficiencies, it is necessary to heat theproduct over the α to (α+γ) phase transformation temperature, and theheat treatment should be performed in the pure α phase area. The α toα+γ phase transformation point depends on the Al content. As far as theTiAl alloy of the invention is concerned, the inventor found fromexperiments that the equation of T(° C.)≧{1200° C.+25(Al−44)}+10 isestablished in this regard.

[0044] After the heat treatment, the product is cooled at a rate of 100°C./min or more. If the cooling speed is set to below 100° C./min,precipitation of γ particles is promoted when passing through the α+γarea during cooling, and layer intervals in the lamellar structure areenlarged. Such microstructural deficiencies are undesirable.

[0045] Now, examples of the present invention and comparative exampleswill be described.

[0046] Referring to Table I and II, prepared were 45 specimens includingAl, Mo (or Fe, or Mo and Fe), V and Si with the remainder being Ti andinevitable impurities in different amounts. Each of these specimens washeated to a melt, and cooled at the cooling rate of 150 to 250° C./minwithin a temperature range of 1500 to 1100° C. The specimens of No. 1 to45 were those as cast. These specimens were evaluated in terms of creepresistance, oxidation resistance and structure observation in thefollowing manners.

[0047] Creep Resistance:

[0048] Each specimen was machined to a rod that has a parallel portionof 6 mm diameter and 30 mm length and subjected to a creep rupture testwith a load of 160 to 270 MPa at a temperature of 760° C. in theatmosphere. The time to rupture (hours) was measured. Major experimentalresults are shown in Tables I and II as well as in FIG. 9. The values inthe “Creep Rupture” columns in Tables I and II indicate those obtainedwhen a load of 240 MPa was applied.

[0049] Oxidation Resistance:

[0050] Each specimen was heated at 800° C. for 30 minutes in athermobalance and cooled to room temperature in 5 minutes. Then, eachspecimen was left alone for 20 minutes and heated to 800° C. again for30 minutes. This cycle was repeated 200 times. One cycle needed 55minutes. Subsequently, the weight change of the specimen was measuredbetween before and after the test. With this result, an amount ofincrease in oxidation per unit area (mg/cm²) was calculated. Majorvalues obtained in this manner were shown in “Oxi. Incr.” columns ofTables I and II as well as in FIG. 10.

[0051] Structure Observation:

[0052] Each specimen was cut, and the resulting (exposed) face wasanalyzed in terms of microstructure by an optical microscope and reflexelectroimage, in order to determine presence/absence of a completelybinary phase lamellar structure, i.e., fully lamellar structure. InTables I and II, O represents existence and X represents absence.

[0053] As understood from FIG. 9, the creep resistance (life) of theinvention TiAl was significantly improved (at least by one digit) overthe conventional TiAl at any stress. As illustrated in FIG. 10, theincrease of oxidation in the invention TiAl was considerably reduced ascompared to the conventional TiAl.

[0054] As shown in Tables I, II and FIGS. 9, 10, the completebinary-phase lamellar structure could not be found in the specimens Nos3, 5 and 7 with the total amount of Mo/Fe, V and Si exceeding 5 at %,and the specimens Nos. 10, 17, 34, 38, 43 and 45 with the sum of Mo andFe exceeding the upper limit of the present invention. On the otherhand, the fully lamellar structure was found in the specimens of theinvention with the total amount of Mo/Fe, V and Si being within 5 at %and the amount of Mo being below the upper limit of the invention.

[0055] The illustrated and described TiAl is disclosed in JapanesePatent Application No. 11-161073 filed on Jun. 8, 1999, the instantapplication claims priority of this Japanese Patent Application, and theentire disclosure thereof is incorporated herein by reference. TABLE I2-Phase Alloy Composition (at %) Creep Layer Speci. Ti + Oxi. Incr. Rup-Struc- No. Al Mo Fe V Si Imp. (mg/cm²) ture ture  1 46.0 3.5 — 0.5 0.5Re- ◯ mainder  2 46.0 3.5 — 1.5 0.5 idem ◯  3 46.0 3.5 — 2.5 0.5 idem X 4 46.0 3.5 0.5 1.5 0.5 idem ◯  5 46.0 3.5 1.0 1.5 0.5 idem X  6 47.03.0 — 1.5 0.3 idem ◯  7 47.0 3.0 0.5 1.5 0.3 idem X  8 47.0 — 0.5 1.00.3 idem 4.6 over ◯ 300 h  9 47.0 — 1.0 1.0 0.3 idem 5.0 over ◯ 300 h 1047.0 2.5 1.2 1.0 0.5 idem X 11 47.5 0.5 — 1.0 0.5 idem 3.3 ◯ 12 47.5 0.5— 1.5 0.3 idem 3.4 ◯ 13 47.5 1.0 — 0.5 0.3 idem 2.8 over ◯ 500 h 14 47.51.0 — 1.0 0.3 idem 3.1 over ◯ 500 h 15 47.5 1.0 — 1.5 0.3 idem 3.4 over◯ 500 h 16 47.5 3.0 — 0.5 0.2 idem ◯ 17 47.5 3.5 — 1.0 0.2 idem X 1848.0 — 0.5 1.0 0.2 idem 4.0 over ◯ 400 h 19 48.0 — 0.8 1.2 0.2 idem 4.6◯ 20 48.0 — 1.2 1.0 0.2 idem 4.8 ◯ 21 48.0 0.5 — 1.0 0.3 idem 2.8 over ◯500 h 22 48.0 0.5 — 1.5 0.3 idem 3.0 over ◯ 500 h 23 48.0 1.5 — 1.0 0.3idem 2.0 over ◯ 500 h 24 48.0 1.5 — 1.5 0.3 idem ◯ 25 48.0 2.5 — 1.0 0.3idem X

[0056] TABLE II 2-Phase Alloy Composition (at %) Creep Layer Speci. Ti +Oxi. Incr. Rup- Struc- No. Al Mo Fe V Si Imp. (mg/cm²) ture ture 26 48.02.5 — 1.0 0.3 Re- ◯ mainder 27 48.0 3.0 — 1.0 0.5 idem ◯ 28 48.0 3.0 1.01.0 0.5 idem X 29 48.5 0.5 — 1.0 0.3 idem 2.8 over ◯ 400 h 30 48.5 0.5 —1.0 0.5 idem 2.6 over ◯ 400 h 31 48.5 1.0 — 0.5 0.3 idem 2.8 over ◯ 400h 32 48.5 1.0 — 0.5 0.5 idem 2.7 over ◯ 400 h 33 48.5 2.0 — 0.5 0.3 idem◯ 34 48.5 3.0 — 0.5 0.5 idem X 35 49.0 0.5 — 0.5 0.5 idem ◯ 36 49.0 1.5— 0.5 0.5 idem ◯ 37 49.0 2.5 — 0.5 0.3 idem ◯ 38 49.0 3.0 — 1.2 0.3 idemX 39 49.0 — 0.5 1.2 0.3 idem ◯ 40 49.0 — 0.5 1.2 0.3 idem ◯ 41 50.0 0.5— 1.5 0.3 idem ◯ 42 50.0 1.5 — 0.5 0.3 idem ◯ 43 50.0 2.5 — 1.2 0.3 idemX 44 50.0 2.0 0.5 1.2 0.3 idem ◯ 45 50.0 2.5 1.0 1.2 0.3 idem X

What is claimed is:
 1. TiAl alloy comprising: Al: 46 to 50 at %; Mo, Vand Si: a total of these elements being 5 at % or less, provided that Sicontent is 0.7 at % or less, and Mo content satisfies an equation of−0.3x+17.5 at % or less where x represents Al (at %); and the remainderbeing Ti and inevitable impurities:
 2. TiAl alloy of claim 1, wherein Alis contained 48±1.0 at %, Mo is contained 0.4 to 0.8 at %, V iscontained 0.5 to 1.1 at %, and Si is contained 0.1 to 0.3 at %.
 3. TiAlalloy comprising: Al: 46 to 50 at %; Fe, V and Si: a total of theseelements being 5 at % or less, provided that Si content is 0.7 at % orless, and Fe content satisfies an equation of −0.3x+17.5 at % or lesswhere x represents Al (at %); and the remainder being Ti and inevitableimpurities.
 4. TiAl alloy of claim 3, wherein Al is contained 48±1.0 at%, Fe is contained 0.4 to 0.8 at %, V is contained 0.5 to 1.1 at %, andSi is contained 0.1 to 0.3 at %.
 5. TiAl alloy comprising: Al: 46 to 50at %; Mo, Fe, V and Si: a total of these elements being 5 at % or less,provided that Si content is 0.7 at %, and a sum of Mo and Fe satisfiesan equation of −0.3x+17.5 at % or less where x represents Al (at %); andthe remainder being Ti and inevitable impurities.
 6. TiAl alloy of claim5, wherein Al is contained 48±1.0 at %, combination of Mo and Fe iscontained 0.4 to 0.8 at %, V is contained 0.5 to 1.1 at %, and Si iscontained 0.1 to 0.3 at %.
 7. A casting method comprising the steps of:A) preparing TiAl alloy having the following composition: Al: 46 to 50at %, Mo, V and Si: a total of these elements being 5 at % or less,provided that Si content is 0.7 at %, and Mo content satisfies anequation of −0.3x+17.5 at % or less where x represents Al (at %), andthe remainder being Ti and inevitable impurities; B) heating the TiAlalloy to a melt; C) pouring the melt into a mold; and D) cooling themelt at a rate of 150 to 250° C./min within a temperature range of 1500to 1100° C. to obtain an as-cast product.
 8. A casting method comprisingthe steps of: A) preparing TiAl alloy having the following composition:Al: 46 to 50 at %, Fe, V and Si: a total of these elements being 5 at %or less, provided that Si content is 0.7 at %, and Fe content satisfiesan equation of −0.3x+17.5 at % or less where x represents Al (at %), andthe remainder being Ti and inevitable impurities; B) heating the TiAlalloy to a melt; C) pouring the melt into a mold; and D) cooling themelt at a rate of 150 to 250° C./min within a temperature range of 1500to 1100° C. to obtain an as-cast product.
 9. A casting method comprisingthe steps of: A) preparing TiAl alloy having the following composition:Al: 46 to 50 at %, Mo, Fe, V and Si: a total of these elements being 5at % or less, provided that Si content is 0.7 at %, and a sum of Mo andFe satisfies an equation of −0.3x+17.5 at % or less where x representsAl (at %), and the remainder being Ti and inevitable impurities; B)heating the TiAl alloy to a melt; C) pouring the melt into a mold; andD) cooling the melt at a rate of 150 to 250° C./min within a temperaturerange of 1500 to 1100° C. to obtain an as-cast product.
 10. The castingmethod of claim 7 further including the step of E) heat treating theas-cast product within a temperature range of 800 to 1100° C.
 11. Thecasting method of claim 10 further including the step of F) cooling theproduct at a rate of 100° C./min or more after step E.
 12. The castingmethod of claim 7, wherein the heat treatment is HIP or homogenization.13. The casting method of claim 8 further including the step of E) heattreating the as-cast product within a temperature range of 800 to 1100°C.
 14. The casting method of claim 13 further including the step of F)cooling the product at a rate of 100° C./min or more after step E. 15.The casting method of claim 13, wherein the heat treatment is HIP orhomogenization.
 16. The casting method of claim 9 further including thestep of E) heat treating the as-cast product within a temperature rangeof 800 to 1100° C.
 17. The casting method of claim 16 further includingthe step of F) cooling the product at a rate of 100° C./min or moreafter step E.
 18. The casting method of claim 16, wherein the heattreatment is HIP or homogenization.
 19. The casting method of claim 7further including the step of E) heat treating the as-cast productwithin a temperature range that satisfies the following equation: T(°C.)≧{1200° C.+25(Al−44)}+10 after step D.
 20. The casting method ofclaim 19 further including the step of F) cooling the product at a rateof 100° C./min or more after step E.
 21. The casting method of claim 19,wherein the heat treatment is HIP or homogenization.
 22. The castingmethod of claim 8 further including the step of E) heat treating theas-cast product within a temperature range that satisfies the followingequation: T(° C.)≧{1200° C.+25(Al−44)}+10 after step D.
 23. The castingmethod of claim 22 further including the step of F) cooling the productat a rate of 100° C./min or more after step E.
 24. The casting method ofclaim 22, wherein the heat treatment is HIP or homogenization.
 25. Thecasting method of claim 9 further including the step of E) heat treatingthe as-cast product within a temperature range that satisfies thefollowing equation: T(° C.)≧{1200° C.+25(Al−44)}+10 after step D. 26.The casting method of claim 25 further including the step of F) coolingthe product at a rate of 100° C./min or more after step E.
 27. Thecasting method of claim 25, wherein the heat treatment is HIP orhomogenization.